A method and apparatus for growing "cultchless" oyster spat is disclosed in U.S. Pat. No 3,526,209 (Budge). The method involves causing free swimming oyster larvae to settle on a relatively smooth surface, such as a sheet made of plastic material. After the larvae have passed through metamorphosis to become oyster spat, they are removed from the sheet to provide "cultchless" oyster spat. These may then be put into tanks where they are maintained in a suspended state either by air-agitation, as disclosed in British Pat. No. 1,366,394 (Scottish Sea Farms Limited), or by upward flow of the culture medium, as disclosed in U.S. Pat. No 3,517,648 (Budge), until they have reached a size large enough to be put into oyster beds for growing to an adult size.
The process of growing "microcultch" spat is described in our copending International Patent Appln No. PCT/AU82/00013. In this process the oyster larvae are induced to settle on small solid particles within a defined particle size range. Definition of the particle size range within close limits allows particles occupied by settled larvae to be separated from unoccupied particles by a simple screening operation. The occupied particles are then maintained in a suspended state in a manner as previously described until the attached spat are 4 mm in size which is large enough for them to be used as seed oysters.
One method of growing "cultchless38 and "microcultch" seed oysters to adult size and cultivating them thereafter to a marketable condition is to enclose them in mesh bags which are then immersed in the sea or estuaries. As the oysters grow so they need to be transferred to bags of successively larger mesh. The largest mesh size which will retain the oysters is desirable in order to obtain maximum accessibility of the sea water to the oysters. Such a method has its shortcomings, such as the high labour involved in rebagging the oysters and the need to remove the fouling materials which grow on and around the bags.